This invention relates to an improved process, and the consequent product, which provides the greatest degree of corrosion protection of flat rolled steel sections, previously galvanized and successively plated with multi-layer electrolytic deposits.
Applicant's copending U.S. patent application Ser. No. 585,856, filed Mar. 2, 1984 U.S. Pat. No. 4,520,077 relates to the electrolytic deposit of one or more layers of inorganic elements or compounds on top of the zinc-base layer constituting the first steel plating layer.
In particular, according to the said process, the electrolytic plating consists of a metal chrome layer and a chrome oxide layer, said plating being obtained by a two-stage electrolytic process carried out continuously in the end portion of a hot galvanizing plant through a dipping or electrogalvanizing plant, wherein zinc or zinc alloys are deposited, whatever the plant characteristics, or even in an autonomous plant, independent of any other plating plant, whether upstream or downstream.
Under these multi-layer plating conditions (zinc or zinc alloys, chrome or chrome oxide) in case of exposure to particularly aggressive environments or solutions, e.g.; acid solutions, there may take place local attacks according to the following mechanism:
After deformations due to the fact that the steel is pressed with a multi-layer plating, crackings of the chrome based layers may form. As a result, the zinc, being the first protection layer, is exposed to environment attacks. The condition is not in itself worrying, both because the multi-layer pre-plated steel is intended for uses involving painting of the goods, and because the zinc itself provides a very good protection action even in the absence of chrome or chrome oxide, and finally because the cracks are microscopic. However, in the event that, before painting, the product made with multi-layer pre-plated steel is submitted to particularly aggressive agents, such as, for instance, acid solutions having a pH lower than 3, the zinc and consequently the plating may be attacked. For instance, such acid solutions may consist in the phosphoric acid solutions used to make an anchoring layer to the successive paint layer, to be applied to a car body. This phosphoric acid treatment provides to be necessary for uncoated sheet-steels. However for economic reasons, in view of the fact that even when pre-coated steel sheets are used of the type specified in U.S. Pat. No. 4,520,077 such multicoated sheets are used only for car parts most vulnerable to environmental attack. Consequently, the phosphoric acid treatment is applied to the body to be painted, including the parts pre-coated according to U.S. Pat. No. 4,520,077. If during forging, microcracks have formed on the said pre-coated pieces uncovering the zinc layer, local corrosion may occur as described hereinafter.
The chromium and chromium oxide layers are particularly resistant to acid agressive agents; therefore, they protect the zinc until there are no microflaws in the said nobler layers. However for a number of reasons (pressing, abrasion, microdefects) there may turn out to be micro-discontinuities in the chromium base layers. When this happens, the highly protective condition may change into a serious acceleration of the zinc dissolution on the part of the aggressive agent; indeed, there may be the well known "pitting" condition, due to the presence of a wide cathodic area (the chrome and chrome oxide plated area) together with a number of tiny anodic areas (those where the zinc is unplated), with the added inconvenience that the aggressive power of the corrosive agent concentrates on the latter, and determines its dissolution, that continues below the areas plated with Cr--CrOx.
Depending on the length of exposition to the aggressive solution, a global attack on the multi-layer plating (Zn--Cr--CrOx), causes the steel to be uncovered in various spots. For instance, a 5 minute dip into a phosphating bath having a pH of 2.5 containing chlorates as accelerators, is enough to cause pitting of the multilayer plating.
In the following specification, as in the preceding one, the words "steel", "zinc-based plating", "galvanized steels" or "zinc-plated steels", "galvanizing" and "multi-layer" have the same meanings as those previously specified and defined in the said U.S. Pat. No. 4,520,077.